The present invention relates to an induction heating device of the type including a switching power source and an image processing device using the same.
An induction heating device of the type described is applicable not only to various furnaces including a metal melting furnace, a plate heating furnace and a hardening furnace, but also to a fixing unit that fixes a toner image on a recording medium in an electrophotographic process. An image processing apparatus may be typified by a copier, a facsimile apparatus and a combination thereof. In a copier, for example, a switching power source often includes a plurality of different lines each including a converter or an inverter. The prerequisite with this kind of switching power source is that sound ascribable to noise interference be obviated. For this purpose, a particular frequency is assigned to each line while a difference in switching frequency between the lines is selected to be higher than an audible range. In practice, however, a low switching frequency must sometimes be used. A transformer included in a line whose switching frequency is low has its iron loss or hysteresis loss aggravated, resulting in a bulky, expensive configuration. Consequently, the switching power source with such a transformer makes the entire device bulky and expensive.
The induction heating device includes an induction coil adjoining a magnetic heating member. A high-frequency current is fed to the induction coil in order to generate a magnetic flux in the heating member. The magnetic flux generates an induced current in a conductive layer formed on the heating member. The resulting Joule heat heats the surface of the heating member to a preselected temperature. To miniaturize the induction heating device and to render the amount of heat adjustable, it is necessary to use a plurality of induction coils or split induction coils and to control each induction coil independently of the others. For this purpose, it is a common practice to use a switching power source for driving the individual induction coil. The switching power source includes a plurality of inverters, or high-frequency power sources, each for controlling a particular induction coil. This, however, brings about a problem that a magnetic flux generated by any one of the induction coils effects the other induction coils. As a result, the inverters interfere with each other and fail to operate.
The following approaches (1) through (3) have been proposed to obviate the interference between the inverters.
(1) The induction coils are positioned remote from each other or isolated from each other by shield plates.
(2) A plurality of induction coils (including split induction coils) are replaced with a single induction coil connected to a single inverter. A gap between the induction coil and a heating element is varied in order to distribute the amount of heat.
(3) A plurality of parallel induction coils are connected to a single large-capacity inverter.
The above approach (1), however, causes irregular heating to occur. The approach (2) cannot cope with a change in the dimension of a heating range or that of an object to be heated. Further, the approach (3) has a problem that a main switching device, constituting the inverter, controls power to be fed to the induction coils, i.e., simply varies the power over all of the induction coils, as distinguished from the individual induction coil. As a consequence, the induction heating device is sophisticated and must have the induction coils to be adjusted, resulting in low reliability. Moreover, the induction heating device is expensive and bulky and has heretofore not been extensively used.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-91260, 9-106207, 9-140135 and 2000-214725.